In e.g. LTE, all uplink transmissions are scheduled by the base station. A grant is transmitted on the downlink control channel PDCCH and the mobile terminal responds with a transmission using the resources specified in the grant and with the size specified in the grant. The mobile terminal can let the base station know that it wants to transmit by sending a scheduling request (SR) on the uplink control channel PUCCH at predefined times. Typically the mobile terminal transmits an SR which is followed by one or many grants, each resulting in one uplink transmission. This is commonly referred to as dynamic scheduling.
It is not required that an SR precedes the grant. When the base station knows that a mobile has a periodic service or for some other reason can predict future data arrivals it can transmit a grant to the mobile without waiting for an SR. It is also possible to blindly transmit grants in order to speed up the scheduling and hence reduce the delay. These scheduling methods are called predictive scheduling.
Discontinuous Reception (DRX) is a mechanism that is specified to save power in a mobile terminal. It allows the terminal to turn off its receiver and transmitter according to predefined rules while maintaining a connection to the base station. The RRC protocol is used to configure DRX cycles and timers that define when the terminal should monitor the downlink control channel PDCCH. When no transmissions are scheduled the mobile wakes up for “on duration” milliseconds during each DRX cycle. There are both long and short DRX cycles. There is also an inactivity timer that keeps the mobile active after the reception of a grant which indicates that data may be transmitted, and the PDCCH is monitored when an ACK/NACK is expected. In addition the mobile terminal is active from the time that an SR is sent until a grant is received. These mechanisms are illustrated in FIG. 1.
Instead of dynamic scheduling, semi-persistent scheduling (SPS) can be used. The purpose with SPS is to save resources on the PDCCH when it is known beforehand when data will arrive to the mobile terminal. When SPS is used, a semi-persistent scheduling interval is signalled to the mobile terminal through the RRC protocol. Special grants (SPS grants/semi-persistent scheduling uplink grants) are then used to configure a recurring grant with the specified interval. One grant can hence be used for multiple transmissions, see FIG. 2. The semi-persistent grant is valid until it is cancelled by a special grant that explicitly releases the semi-persistent grant. To optimize the power saving with SPS, a mechanism has been added so that no SR is triggered by specified logical channels while an SPS grant is configured.